A fun way to spend $600 million that you cannot launder anyway would be to put it in a smart contract and release it in portions to anyone who could solve a proof of work problem. Incidentally, the PoW problem would result in empty blocks being mined on Bitcoin, effectively executing a denial of service attack. The incentives should be adjusted so it becomes at least twice as profitable for a miner to mine empty blocks for the smart contract than to mine real blocks on the Bitcoin main network.

Edit: It should probably be designed as a contract which pays out only once a predetermined number of fake blocks have been mined, and the payout to the solvers will be proportional to the number of blocks they have in the final solution chain (wallet addresses of solvers should be encoded in the empty blocks they mine to prevent front-running, which will destroy the incentives). It should be possible to submit any solution to the contract so that everyone competes. This includes solutions that are mined by actual bitcoin miners, effectively pitting the miners mining real blocks against the miners mining empty blocks. Someone who is more skilled than me should design such a contract and put it on Ethereum, ready to receive Ether that someone "finds".